The present invention relates to push-pull slip sheet handling attachments for forklift trucks and especially to such attachments that are fork-supported.
Traditionally, packaged or containerized loads have been stacked on wooden pallets for handling by high-lift vehicles such as forklift trucks and self-propelled or manually-operated low-lift vehicles known as pallet trucks, hand jacks or "walkies." The conventional narrow forks of such vehicles are inserted into openings in the pallets for lifting, transporting and stacking or unstacking the loads.
In the last decade, thin fiber or plastic slip sheets have been used increasingly instead of pallets to handle loads because of their greater convenience and lower cost. However, high-lift and low-lift vehicles equipped with standard, narrow forks (typically 4 to 6 inches wide) for handling pallets are inadequate for handling loads on slip sheets because of the small load-supporting surface area of the forks. Therefore, such vehicles have been equipped with wide forks or platens to support slip-sheeted loads instead of conventional narrow pallet forks. In addition, such vehicles must be equipped with a push-pull attachment which typically includes a push plate connected to a rear frame by a powered mechanism for extending and retracting the push plate above the forks or platen. A powered gripping jaw mechanism attached to the bottom of the push plate grips a free edge of a loaded slip sheet for pulling it onto the platen or wide forks.
A forklift truck equipped with a push-pull attachment and wide forks for handling slip sheets as described is shown, for example, in Brudi U.S. Pat. No. 3,640,414. Low-lift, walkie-type vehicles normally used for handling pallets but adapted to handling slip sheets by the addition of wide forks or platens and push-pull mechanisms are shown, for example, in Rocco U.S. Pat. No. 4,065,012, Olson U.S. Pat. No. 4,274,794 and Frees U.S. Pat. No. 4,300,867.
The primary drawback of the described slip sheet attachments for forklift trucks is that they require the removal of the conventional narrow lift truck forks and their replacement with wide forks or platens and the addition of the push-pull assembly. This is both laborious and time-consuming, especially in the many warehouses, shipping terminals, distribution centers, and other facilities that must be equipped to handle both slip-sheeted and palletized loads. In such facilities, either the conversion of a truck from slip sheets to pallets or vice versa must occur repeatedly, resulting in lost time, or the facility must purchase additional vehicles and equip them for handling only pallets or only slip sheets. Both solutions are undesirable because ultimately they are costly. The problems in converting low-lift vehicles from pallet-handling to slip sheet handling are comparable.
Confronted with the foregoing dilemma, others have developed slip sheet handling attachments which can be quickly attached to and detached from forklift trucks and low-lift vehicles while the conventional narrow pallet-handling forks remain on the vehicle.
One such attachment for a forklift truck has been developed and marketed by DF Industries, Inc., of Alpharetta, Ga. (DF attachment). This unit includes a single wide platen connected to an upright rear frame. The frame mounts a push-pull assembly including a hydraulically powered horizontal pantograph mechanism which moves a push plate with slip sheet gripping jaws in and out over the platen. The entire unit rests on and is fully supported on the upper surfaces of the conventional narrow forks. The rear frame connects directly to the forks by connecting pins which extend behind the upright portions of the fork heels to retain the attachment on the truck. The DF attachment has a self-contained hydraulic power unit which draws electric power from the lift truck. The attachment is quite lightweight, being made mostly of aluminum, but because of this is vulnerable to damage from the abuse to which such attachments are commonly subjected in normal industrial use.
One version of the DF attachment is adapted for mounting on a walkie or hand jack. This version adds to the forklift version a powered push-pull cylinder which connects the entire unit to the low-lift vehicle frame so that the entire unit, including push-pull frame and platen, can extend and retract on the conventional forks of such vehicles. When extended, the platen tips down to ground level to enable the gripping jaws of the push plate to grip a ground-level slip sheet and pull it onto the platen, after which retraction of the push-pull cylinder pulls the unit onto the forks.
More recently, Cascade Corporation of Portland, Oreg., has also marketed a push-pull slip sheet attachment (Cascade attachment) which can be mounted on a forklift truck without removing the conventional narrow pallet forks. An attachment substantially similar to the Cascade attachment as marketed is shown in Farmer, et al. U.S. Pat. No. 4,482,286 (Farmer patent), and unless otherwise noted, both are referred to herein as the Cascade attachment. Rather than connecting directly to the forks of a lift truck as in the DF attachment, the attachment of the Farmer patent connects to the lower cross bar, and the Cascade attachment as marketed connects to both the upper and lower cross bars, of an Industrial Truck Association (ITA) standardized lift truck carriage.
The Cascade attachment has dual platens, which, like the single platen of the DF attachment, are vertically supported on the forks, at least when the platens are loaded. The rear frame of the Cascade attachment nests between the fork uprights, rather than in front of them as in the DF attachment. The Cascade attachment, unlike the DF attachment, does not have a self-contained hydraulic power unit. Instead, like other push-pull attachments, it takes its hydraulic power from the lift truck through hydraulic hoses, which supply a pair of push-pull cylinders and a pair of gripping jaw cylinders on the attachment. As a result of these and other refinements, the Cascade attachment with its push-pull assembly fully retracted, has substantially less fore and aft bulk or thickness than the DF attachment, and a center of gravity closer to the front wheels of a connected truck. Nevertheless, the Cascade attachment is considerably heavier than the DF attachment, primarily because of its nearly all-steel construction and its heavy frame design.
One of the great concerns with any lift truck attachment is the extent to which it reduces the net load capacity of the lift truck on which it is used. The rated load capacity of most lift trucks is based on use of the truck with conventional narrow forks. It is measured as that load which the truck can handle at a distance of 24 inches from the outwardly facing surface of the fork upright portions with the conventional forks attached to the carriage. Generally, when an attachment which is heavier than the conventional forks replaces those forks on a lift truck, the net load capacity of the truck is reduced. Similarly, when an attachment is added to a lift truck on top of the forks, the added weight of the attachment reduces the net load capacity of the truck.
Typically, lift truck attachments, because of their bulk, also reduce the net load capacity of a lift truck by shifting the center of gravity of a load carried by the truck forward from where it would be if handled solely by the truck's conventional forks. This center of gravity-shifting of the load by the attachment is referred to in the industry as the "lost load" or "effective thickness."
Accordingly, three characteristics of an attachment reduce the rated net load capacity of a forklift truck. These are (1) its weight; (2) the location of its center of gravity or mass; and (3) its lost load or effective thickness. Good attachment design dictates that all three of these factors be minimized to keep the fork truck net capacity with the attachment as close as possible to the net capacity of the truck without the attachment. A major disadvantage of the prior fork-supported push-pull attachments described is that they reduce to an undesirable extent the net load capacity of a lift truck, either because they are too heavy or because they have large lost loads.
An additional problem can arise in the use of fork-supported push-pull attachments which use top hooks to connect the attachment to the upper cross bar of the ITA carriage of a lift truck, a common practice. The upright rear frame of the attachment is generally at right angles to the platens, and the top hooks mount the rear frame against the face of the ITA carriage. As a result, the platens will not seat fully on the forks when unloaded unless the upright and horizontal tine portions of both forks also meet at right angles. However, forks typically become bent in use so that their upright and tine portions meet at greater than 90.degree.. Also, the two forks on a truck usually define different angles. To further complicate the problem, the tines of some forks have top-surface tapers instead of the usual bottom-surface taper from heel to tip, in which case their initial inside angles are greater than 90.degree..
Placing the foregoing in perspective, a 1.degree. top taper or bend deviation from 90.degree. on a 42 inch fork tine can result in a 3/4 inch gap between the fork tip and bottom of the platen. This leads to practical problems for the lift truck operator in handling loads. For example, a typical use of a push-pull attachment is to insert the platen between stacked loads and pull the top load onto the platens. A gap between the fork tip and platen complicates and slows this procedure for the lift truck operator and causes bottom load damage from the forks if care is not exercised, placing an undesirable burden on the operator.
Another problem in the use of prior fork-supported slip sheet handling attachments is the permanent bending of the platens that can occur with the thin steel platens typically used. Such attachments usually have thin platens to save weight, relying on the underlying forks to provide the necessary stiffness to handle loads. However, the forks alone have been inadequate to this task because the platens usually extend beyond the tips of the forks a substantial distance and also laterally well beyond the forks in one or both directions. Thus, the platens are typically weak in bending, especially near their tips. Lift truck operators, using such attachments in scooping under loads, often attempt to lift a load at the unsupported tips of the thin platens, causing the platens to take a permanent set. To avoid such bending problems, thicker platens have been used, but this adds undesirable weight to the attachment and shifts the center of gravity further forward from the carriage, thereby further reducing the net load capacity of the lift truck.
One version of the DF attachment incidentally avoids most platen bending problems through the use of fork-receiving pockets on the underside of the platens. The primary purpose of such fork pockets is to mount the attachment on the forks rather than on the ITA carriage. However, because of the wide variation in the size and shape of forks commonly in use, fork pockets cannot be designed to accomodate all such forks without adding excessive weight to the attachment.
A disadvantage of prior fork-supported push-pull attachments of the type shown in the Farmer patent is that they can be attached only to lift trucks equipped with ITA lift carriages characterized by the types of upper and lower fork-connecting cross bars shown in such patent. The forks for an ITA carriage have hooks which hook to the upper and lower cross bars of the carriage. Other attachments, including push-pull attachments, for such a carriage typically include similar hooks for the same purpose. However, many forklift trucks, especially older ones, do not use ITA lift carriages to connect the forks to the truck. Instead, they use so-called "pin-type" carriages whereby the forks are pivoted relative to the carriage on a pin or shaft Such carriages do not have the equivalent of the ITA cross bars and therefore do not accept forks or other attachments having hook-type connectors. Accordingly, attachments of the type shown in the Farmer patent cannot be used on a lift truck with a pin-type carriage.
Although an attachment of the DF type can be mounted on lift trucks having both ITA and pin-type carriages because the DF attachments connect to the forks not the carriage, the DF pin-type connector for this purpose has some serious disadvantages. When a DF type attachment is fitted to pin-type forks of a lift truck having a tilt mechanism, the attachment must be spaced a substantial distance in front of the fork uprights to clear the tilt mechanism. This spacing is accomplished by spacer bolts on the frame. These same spacer bolts are used to snug the fork connecting pins against the backs of the forks on both ITA and pin-type carriages, both of which commonly carry forks of different thicknesses. The use of such spacers increases the lost load of the attachment, reducing the net load capacity of the truck. Such spacers also require the use of tools to make the adjustment, which is especially disadvantageous in those facilities in which unions require mechanics to make equipment adjustments when tools are required. The use of both a mechanic and a forklift truck driver to connect and disconnect attachments from lift trucks is inefficient.
The connecting means for connecting prior fork-supported attachments to lift trucks have been subject to damage and breakage under normal industrial use. One problem is that the quick-connect elements of such connecting means have been located in close proximity to the forks, subjecting them to fork impact as the operator attempts to insert the forks beneath the platens during installation of the attachment on a truck. Some such connecting means have also included elements which become broken or damaged by catching on cracks or floor protrusions such as loading ramps as the fork truck is driven over them with the load positioned close to the floor as is commonly done for safety.
Prior fork-supported push-pull attachments are not self-supporting in a stable free-standing position on a level ground surface when detached from a lift truck such that the forks of a lift truck can be inserted beneath and withdrawn from beneath the attachment while in such position. For example, in the attachment of the Farmer patent, a pivoting hook-type connector swings down and forward to a frame support position extending below the lower limits of the forks so that, theoretically, the lower connecting hooks and tips of the platens support the attachment in a free-standing position on level ground when detached from a lift truck. However, when thus supported with its push-pull mechanism retracted, the attachment is unstable because its center of gravity is so far above the platens and so close to the rear of the attachment in relation to the rear support point provided by the hooks that the attachment will tend to tip over backward with a small push in that direction. This is obviously undesirable for safety and other reasons in an industrial environment.
Although the center of gravity of such an attachment might be shifted forward by extending the push plate when the attachment is free-standing, this solution tends to bend the platens making it difficult to insert the forks under the attachment for installation. There is also the possibility that lift truck operators would forget to extend the push plate when detaching the unit from a truck. In practice, the instability problem has been solved by elevating the attachment on a large wood block or frame rather than setting it directly on the floor. In an industrial environment, this is an inconvenience because the block can get misplaced, the attachment must be deposited in a designated area where the blocks are located, and the block raises the sharp platen tips several inches off the floor, which is also unsafe. The foregoing instability problems are increased if the platens are detached from the frame with the attachment in a free-standing mode.
Accordingly, there is still a need for a quick-mounting fork-supported slip sheet handling attachment for a forklift truck which (a) minimizes the reduction in rated load capacity of the truck; (b) is easily adaptable for connection to trucks having both ITA and pin-type lift carriages; (c) has quick-connect means capable of withstanding the rigors of industrial use with all types of forks in common use; (d) has platens that are lightweight yet strong to resist permanent bending under normal industrial use; (e) is stable in a self-supporting, free-standing mode with or without the platens and irrespective of push plate position; and (f) can easily be attached to a lift truck from its free-standing position.
Primary objectives of the present invention, therefore, are to provide a new and improved fork-supported slip sheet handling attachment for a forklift truck that:
(1) minimizes reduction in the rated net load capacity of the truck by being lightweight and having a minimum lost load or effective thickness;
(2) is adapted for connection to lift trucks having both pin-type and ITA-type fork carriages and forks of different sizes and shapes;
(3) is quickly and easily attachable to and detachable from the forklift truck;
(4) is easy to mount on and remove from the forks of a lift truck with the truck operator at his controls;
(5) is self-supporting in an elevated free-standing mode when removed from a lift truck to facilitate its rapid mounting on and removal from the forks of a lift truck with the truck operator at his controls; and
(6) is stable when in its free-standing mode, even with its platens removed; and
(7) is strong and durable despite its light weight.